This invention relates to immersion pyrometers and more particularly to structures for protecting temperature sensing elements in pyrometers used to measure the temperatures of molten metals.
Many industrial and scientific processes require the measurement and control of extremely high temperatures. For example, measurements of the temperature of molten metals are essential to proper process control in the metal processing industry. Two of the most common instruments used to determine the temperatures of molten metals are the optical pyrometer and the disposable lance thermocouple. However, each of these devices has its disadvantages. The optical pyrometer is not as accurate as is desirable, and can only measure the surface temperature of the molten metal. The disposable lance thermocouple is inaccurate, does not permit continuous measurement of the temperature of the molten metal, and its use involves some safety problems for the person using it.
As a result of the shortcomings of the optical pyrometer and the disposable lance thermocouple, considerable effort has been expended in developing an immersion pyrometer which has a long-term continuous reading capability. In one type of an immersion pyrometer, a thermocouple junction is encased in a tube made of a metal with a high melting temperature which is coated with a ceramic, such as Al.sub.2 O.sub.3 or a mixture of Al.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 which protects the metal tube from the molten metal environment. However, in use the ceramic layer or layers tends to spall and permit molten metal to contact the metal substrate and attack it. The inner metal tube cannot withstand attack by the slag and/or the molten metal and it, together with the sensing element enclosed therein is quickly destroyed. The sensing element, usually a noble metal thermocouple, is expensive and it is desirable to be able to reuse it many times. However, structures which have been designed to protect the thermocouples have resulted in a slow thermal response, making them substantially ineffective for many purposes.
U.S. patent application Ser. No. 775,183, filed Sept. 12, 1985, now U.S. Pat. No. 4,721,534 for IMMERSION PYROMETER describes a protective sheath for a thermocouple which is capable of withstanding severe thermal shock and which exhibits good resistance to corrosion and erosion by molten metals. While this sheath has an service life which is relatively long, it is desirable to increase the service life even further. It is especially desirable to increase the ability of a pyrometer to withstand a preheat cycle in an oxidizing atmosphere, and to mount the sheath in a fixed position so that it remains in molten metal and out of contact with slag throughout the vessel campaign.